The present invention relates to large surface area geogrids with a high tensile strength, a method and apparatus for producing them, and their use as drain and reinforcement grids.
Geogrids of this nature are used, for example, to secure road and rail structures, to secure earth, to stabilize slopes and to secure landfill sealing systems.
The so-called Tensar(copyright) geogrids produced by Netlon have been in use throughout the world in a very wide range of application areas since as early as the late 1970s.
To produce geogrids of this nature, extruded polyethylene or polypropylene webs are perforated at regular intervals. While being heated, the webs are stretched either in the longitudinal direction (uniaxially) as described in British Patent 2,073,090 or in the longitudinal and transverse directions (biaxially) as described in British Patent 2,035,191. The stretching brings the polymer molecules of a randomly arranged layer into an ordered and aligned position in the direction of stretching. This method increases the tensile strength and the rigidity of the geogrids. A further development to these geogrids is described in U.S. Pat. No. 4,618,385 (Mercer). However, these geogrids have the problem that the grid points cannot be stretched uniformly in the same way as the webs running between the grid points, so that with grids which have been stretched in this way the strength with respect to the weight per square meter is to a certain extent unsatisfactory.
In order to improve the ratio of strength to weight per square meter, DE-C 41 37 10 (Akzo) has described a method for producing geogrids in which firstly strips comprising two layers of polymers which have different melting ranges are produced and then stretched (molecular-oriented bicomponent strips). Then, the strips are laid crosswise in rows, in such a manner that the side of the strips which has the lower melting range rests against another such side. The resultant structure is then exposed to a temperature which is above the melting range of the polymer with the lower melting range but below the melting range of the polymer with the higher melting range. As a result, the intersections of the strips of adjacent rows are joined together via the polymer with the low melting range.
British Patent Application 2,314,802 (Mercer) is based on a similar method. In this document, the introduction to the description states, with regard to the prior art, that the Signode company produces geogrids made from molecular-oriented polyester ribbons which are coated on one side with a plastic which has a lower melting point (bicomponent ribbons). These bicomponent polyester ribbons are then placed crosswise on top of one another in such a way that those sides which have a low melting point bear against one another in the crossing areas. Then, the crossing areas are welded.
The drawback of these geogrids is that the strength of the join in the crossing areas, which is predetermined by the lower-melting polymer component, is unsatisfactory.
To eliminate this drawback, the above mentioned British Patent Application 2,314,802 (applied for on Jul. 2, 1996 and published on Jan. 14, 1998) has developed a method in which molecular-oriented bicomponent strips are also used, but with the modification that one bottom bicomponent strip and one top bicomponent strip per grid web are positioned in the direction of the machine, specifically in such a way that those sides of the two strips which have the lower melting point rest on top of one another over their entire surfaces after the transverse strips have been introduced. Then, in each case the bottom bicomponent strips, with the inclusion of the transverse strips, are joined to the top bicomponent strips over their entire surfaces by means of flame welding or hot-air welding.
Although this method does increase the strength of the join in the crossing area, it has the drawback that, considered from a materials perspective, two different polymers are required in order to produce the bicomponent strips and in each case two bicomponent strips are required to form the corresponding web component.